The present invention relates generally to low frequency, or "voiceband", wire transmission systems, and more particularly to a family of improved 2-wire multiport resistance bridges, or branching networks, for interconnecting a plurality of stations or devices in such a system.
In addition to their role in providing and maintaining the nationwide public telephone system, the Bell System and other telephone operating companies furnish private line circuits for various applications. Examples include circuits used for private voice communications, digital data transmission, security or fire alarm monitoring, and the like. Such circuits typically have a relatively small number of interconnected lines or stations (compared with a public communications circuit), and are specifically engineered for the particular service.
The various "legs" of a multistation private line circuit are interconnected in the telephone company central office by means of a resistance bridge that provides the proper terminating inpedance (typically 600 ohms) for each of the connected legs. Depending on the particular application, each leg may consist of a single pair of wires or two associated pairs, the latter arrangement being used to provide separate transmission paths for each direction of communication. Circuits of the 2-wire type, and some of the 4-wire variety, are formed using 2-wire bridges to interconnect the legs--an arrangement referred to as "split bridge" operation in the case of a 4-wire circuit. FIG. 1 of the accompanying drawing shows a conventional 2-wire, N-port bridge in schematic form. The port-to-port loss of such a bridge varies according to the maximum number of legs it is designed to accomodate. For example, a conventional 2-wire, 4-port bridge for 600 ohm lines has a natural insertion loss of about 9.5 dB, while the port-to-port loss of a similar 12-port bridge is about 20.8 dB and that of a 15-port bridge is 22.9 dB. On standard toll lines, signals are received at +7 dBm and sent out at -16 dBm, a power difference of 23 dB from input to output. Accordingly, when a 2-wire bridge with fewer than 15 ports is used, additional attenuation or "padding" must be added to the circuit to match the standardized power levels. This is typically done by connecting the bridge to an external attenuating pad with jumper wires.
In U.S. Pat. No. 4,140,875, issued Feb. 20, 1979, it is pointed out that no external padding is required for a 4-wire, 8-port 600 ohm bridge. Such a bridge has a natural loss of about 22.6 dB, and the additional loss needed to reach the desired 23.0 dB level normally is accounted for by the central office cabling. Significant economic and other benefits would be realized by eliminating the need to use external pads with 2-wire bridges of fewer than 15 ports, especially the common 4-, 8- and 12-port sizes, when setting up multistation private line circuits. Accordingly, a primary object of the present invention is to provide a family of improved 2-wire, N-port resistance bridges (N being less than 15) having an intrinsic loss sufficiently high that no additional padding is needed to achieve a line input-to-output attenuation ratio of 23 dBm.
A more specific object of the invention is to provide a family of 2-wire, N-port bridges having an intrinsic insertion loss of about 22.6 dB.
Another object of the invention is to provide 2-wire, N-port bridges that are less sensitive to inaccuracies in port terminations.
Still another object is to provide 2-wire multiport bridges in which unused ports need not be terminated with the characteristic impedance of the bridge to achieve satisfactory results.
The true scope of the present invention is set forth with particularity in the appended claims. However, the various objects, features and advantages of the invention will be better understood and appreciated by reference to the accompanying drawing and the following description of the best mode contemplated for its practice.